cue/load/internal/...: copy pkgs from Go source

These packages are directly taken without any modifications (other than
import path rewriting, and some copying of the atomic.Bool code for
go1.18 compatibility) from the Go source tree inside cmd/go/internal,
at commit a4d5fbc3a48b63f19fcd2a4d040a85c75a2709b5.

These will be used by the module loading code.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I83f846e1d146d279d38d566736ad2c081e81ce54
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/549760
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cue/load/internal/mvs/errors.go

@@ -0,0 +1,103 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mvs
+
+import (
+	"fmt"
+	"strings"
+
+	"golang.org/x/mod/module"
+)
+
+// BuildListError decorates an error that occurred gathering requirements
+// while constructing a build list. BuildListError prints the chain
+// of requirements to the module where the error occurred.
+type BuildListError struct {
+	Err   error
+	stack []buildListErrorElem
+}
+
+type buildListErrorElem struct {
+	m module.Version
+
+	// nextReason is the reason this module depends on the next module in the
+	// stack. Typically either "requires", or "updating to".
+	nextReason string
+}
+
+// NewBuildListError returns a new BuildListError wrapping an error that
+// occurred at a module found along the given path of requirements and/or
+// upgrades, which must be non-empty.
+//
+// The isVersionChange function reports whether a path step is due to an
+// explicit upgrade or downgrade (as opposed to an existing requirement in a
+// go.mod file). A nil isVersionChange function indicates that none of the path
+// steps are due to explicit version changes.
+func NewBuildListError(err error, path []module.Version, isVersionChange func(from, to module.Version) bool) *BuildListError {
+	stack := make([]buildListErrorElem, 0, len(path))
+	for len(path) > 1 {
+		reason := "requires"
+		if isVersionChange != nil && isVersionChange(path[0], path[1]) {
+			reason = "updating to"
+		}
+		stack = append(stack, buildListErrorElem{
+			m:          path[0],
+			nextReason: reason,
+		})
+		path = path[1:]
+	}
+	stack = append(stack, buildListErrorElem{m: path[0]})
+
+	return &BuildListError{
+		Err:   err,
+		stack: stack,
+	}
+}
+
+// Module returns the module where the error occurred. If the module stack
+// is empty, this returns a zero value.
+func (e *BuildListError) Module() module.Version {
+	if len(e.stack) == 0 {
+		return module.Version{}
+	}
+	return e.stack[len(e.stack)-1].m
+}
+
+func (e *BuildListError) Error() string {
+	b := &strings.Builder{}
+	stack := e.stack
+
+	// Don't print modules at the beginning of the chain without a
+	// version. These always seem to be the main module or a
+	// synthetic module ("target@").
+	for len(stack) > 0 && stack[0].m.Version == "" {
+		stack = stack[1:]
+	}
+
+	if len(stack) == 0 {
+		b.WriteString(e.Err.Error())
+	} else {
+		for _, elem := range stack[:len(stack)-1] {
+			fmt.Fprintf(b, "%s %s\n\t", elem.m, elem.nextReason)
+		}
+		// Ensure that the final module path and version are included as part of the
+		// error message.
+		m := stack[len(stack)-1].m
+		if mErr, ok := e.Err.(*module.ModuleError); ok {
+			actual := module.Version{Path: mErr.Path, Version: mErr.Version}
+			if v, ok := mErr.Err.(*module.InvalidVersionError); ok {
+				actual.Version = v.Version
+			}
+			if actual == m {
+				fmt.Fprintf(b, "%v", e.Err)
+			} else {
+				fmt.Fprintf(b, "%s (replaced by %s): %v", m, actual, mErr.Err)
+			}
+		} else {
+			fmt.Fprintf(b, "%v", module.VersionError(m, e.Err))
+		}
+	}
+	return b.String()
+}

cue/load/internal/mvs/graph.go

@@ -0,0 +1,225 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mvs
+
+import (
+	"fmt"
+
+	"cuelang.org/go/cue/load/internal/slices"
+
+	"golang.org/x/mod/module"
+)
+
+// Graph implements an incremental version of the MVS algorithm, with the
+// requirements pushed by the caller instead of pulled by the MVS traversal.
+type Graph struct {
+	cmp   func(v1, v2 string) int
+	roots []module.Version
+
+	required map[module.Version][]module.Version
+
+	isRoot   map[module.Version]bool // contains true for roots and false for reachable non-roots
+	selected map[string]string       // path → version
+}
+
+// NewGraph returns an incremental MVS graph containing only a set of root
+// dependencies and using the given max function for version strings.
+//
+// The caller must ensure that the root slice is not modified while the Graph
+// may be in use.
+func NewGraph(cmp func(v1, v2 string) int, roots []module.Version) *Graph {
+	g := &Graph{
+		cmp:      cmp,
+		roots:    slices.Clip(roots),
+		required: make(map[module.Version][]module.Version),
+		isRoot:   make(map[module.Version]bool),
+		selected: make(map[string]string),
+	}
+
+	for _, m := range roots {
+		g.isRoot[m] = true
+		if g.cmp(g.Selected(m.Path), m.Version) < 0 {
+			g.selected[m.Path] = m.Version
+		}
+	}
+
+	return g
+}
+
+// Require adds the information that module m requires all modules in reqs.
+// The reqs slice must not be modified after it is passed to Require.
+//
+// m must be reachable by some existing chain of requirements from g's target,
+// and Require must not have been called for it already.
+//
+// If any of the modules in reqs has the same path as g's target,
+// the target must have higher precedence than the version in req.
+func (g *Graph) Require(m module.Version, reqs []module.Version) {
+	// To help catch disconnected-graph bugs, enforce that all required versions
+	// are actually reachable from the roots (and therefore should affect the
+	// selected versions of the modules they name).
+	if _, reachable := g.isRoot[m]; !reachable {
+		panic(fmt.Sprintf("%v is not reachable from any root", m))
+	}
+
+	// Truncate reqs to its capacity to avoid aliasing bugs if it is later
+	// returned from RequiredBy and appended to.
+	reqs = slices.Clip(reqs)
+
+	if _, dup := g.required[m]; dup {
+		panic(fmt.Sprintf("requirements of %v have already been set", m))
+	}
+	g.required[m] = reqs
+
+	for _, dep := range reqs {
+		// Mark dep reachable, regardless of whether it is selected.
+		if _, ok := g.isRoot[dep]; !ok {
+			g.isRoot[dep] = false
+		}
+
+		if g.cmp(g.Selected(dep.Path), dep.Version) < 0 {
+			g.selected[dep.Path] = dep.Version
+		}
+	}
+}
+
+// RequiredBy returns the slice of requirements passed to Require for m, if any,
+// with its capacity reduced to its length.
+// If Require has not been called for m, RequiredBy(m) returns ok=false.
+//
+// The caller must not modify the returned slice, but may safely append to it
+// and may rely on it not to be modified.
+func (g *Graph) RequiredBy(m module.Version) (reqs []module.Version, ok bool) {
+	reqs, ok = g.required[m]
+	return reqs, ok
+}
+
+// Selected returns the selected version of the given module path.
+//
+// If no version is selected, Selected returns version "none".
+func (g *Graph) Selected(path string) (version string) {
+	v, ok := g.selected[path]
+	if !ok {
+		return "none"
+	}
+	return v
+}
+
+// BuildList returns the selected versions of all modules present in the Graph,
+// beginning with the selected versions of each module path in the roots of g.
+//
+// The order of the remaining elements in the list is deterministic
+// but arbitrary.
+func (g *Graph) BuildList() []module.Version {
+	seenRoot := make(map[string]bool, len(g.roots))
+
+	var list []module.Version
+	for _, r := range g.roots {
+		if seenRoot[r.Path] {
+			// Multiple copies of the same root, with the same or different versions,
+			// are a bit of a degenerate case: we will take the transitive
+			// requirements of both roots into account, but only the higher one can
+			// possibly be selected. However — especially given that we need the
+			// seenRoot map for later anyway — it is simpler to support this
+			// degenerate case than to forbid it.
+			continue
+		}
+
+		if v := g.Selected(r.Path); v != "none" {
+			list = append(list, module.Version{Path: r.Path, Version: v})
+		}
+		seenRoot[r.Path] = true
+	}
+	uniqueRoots := list
+
+	for path, version := range g.selected {
+		if !seenRoot[path] {
+			list = append(list, module.Version{Path: path, Version: version})
+		}
+	}
+	module.Sort(list[len(uniqueRoots):])
+
+	return list
+}
+
+// WalkBreadthFirst invokes f once, in breadth-first order, for each module
+// version other than "none" that appears in the graph, regardless of whether
+// that version is selected.
+func (g *Graph) WalkBreadthFirst(f func(m module.Version)) {
+	var queue []module.Version
+	enqueued := make(map[module.Version]bool)
+	for _, m := range g.roots {
+		if m.Version != "none" {
+			queue = append(queue, m)
+			enqueued[m] = true
+		}
+	}
+
+	for len(queue) > 0 {
+		m := queue[0]
+		queue = queue[1:]
+
+		f(m)
+
+		reqs, _ := g.RequiredBy(m)
+		for _, r := range reqs {
+			if !enqueued[r] && r.Version != "none" {
+				queue = append(queue, r)
+				enqueued[r] = true
+			}
+		}
+	}
+}
+
+// FindPath reports a shortest requirement path starting at one of the roots of
+// the graph and ending at a module version m for which f(m) returns true, or
+// nil if no such path exists.
+func (g *Graph) FindPath(f func(module.Version) bool) []module.Version {
+	// firstRequires[a] = b means that in a breadth-first traversal of the
+	// requirement graph, the module version a was first required by b.
+	firstRequires := make(map[module.Version]module.Version)
+
+	queue := g.roots
+	for _, m := range g.roots {
+		firstRequires[m] = module.Version{}
+	}
+
+	for len(queue) > 0 {
+		m := queue[0]
+		queue = queue[1:]
+
+		if f(m) {
+			// Construct the path reversed (because we're starting from the far
+			// endpoint), then reverse it.
+			path := []module.Version{m}
+			for {
+				m = firstRequires[m]
+				if m.Path == "" {
+					break
+				}
+				path = append(path, m)
+			}
+
+			i, j := 0, len(path)-1
+			for i < j {
+				path[i], path[j] = path[j], path[i]
+				i++
+				j--
+			}
+
+			return path
+		}
+
+		reqs, _ := g.RequiredBy(m)
+		for _, r := range reqs {
+			if _, seen := firstRequires[r]; !seen {
+				queue = append(queue, r)
+				firstRequires[r] = m
+			}
+		}
+	}
+
+	return nil
+}